Control Systems in the Living World - Building the Most Complex Structure on Earth - page 25

Metaphase Telophase

Anaphase

Spindle midzone

Mitotic spindle

Contractile ring

Interpolar

Centrosome Microtubule Central spindle Astral

microtubule microtubules Midbody

Kinetochore

Figure 1.13 Assembly of the central spindle. Schematic diagrams of the distribution of
microtubules and the chromosomes during cell division. In metaphase, the chromosomes
align on the metaphase plate. At anaphase, the chromosomes move poleward, the central
spindle assembles, and contractile ring assembly commences. In telophase, after cleavage
furrow ingression, the contractile ring compresses the central spindle to form the midbody.
Microtubule plus (+) ends are indicated (minus ends, which are positioned at the centrosomes,
are not shown) ( Glotzer, 2009 ).

during segregation and so that the appropriate genes are accessible to polymerases
in the emergent mother and daughter cell” ( Bloom and Joglekar, 2010 ). The machin-
ery for the separation of sister chromatids and their transport to the daughter cells is
the spindle apparatus consisting of the two microtubule-organizing centers (MTOCs)
arising from the duplication of interphase centrosomes that, after detaching from the
cell nucleus, relocate in one of the opposing poles of the cell. Microtubules, which
are nucleated in centrioles, extend from spindle poles in the direction of the equato-
rial plate at which they attach to kinetochores. Spindles position themselves by astral
microtubules nucleated from both of their poles and are attached to cortical sites
( Glotzer, 2009; Grill and Hyman, 2005 ).
The force necessary for the segregation of sister chromatids is provided by motor
proteins and by gradual, controlled depolymerization of the bipolar mitotic spin-
dle microtubules. Kinetochores of sister chromatids are attached to microtubules at
opposing poles, and hence, they move in opposing directions to form the complete
set of chromosomes to each daughter cell ( Figure 1.13 ). Almost all the microtubule-
based force for the transport of chromosomes is provided by motor proteins, and
microtubules determine the correct positioning of chromosomes in daughter cells:
“The mitotic spindle supplies force, as well as positional cues, to the chromosome so
that chromosome movements are consistent with the geometry of the dividing cell”
( Bloom and Joglekar, 2010 ). The elongation of the mitotic spindle during anaphase
leads to the formation of the central spindle, an array of antiparallel microtubules
that is primarily responsible for cytokinesis ( Glotzer, 2009 ).
Two decades ago, eukaryote cytoskeletal proteins (microtubules, actin filaments,
and intermediate filaments) were discovered in prokaryotes. Soon after that, it was
observed that the movement of daughter nucleoids (DNA-containing regions in
prokaryotes) in two opposing poles requires the prokaryote actin homologue MreB
( Gerdes et al., 2004 ) and β-tubulin homologue FtsZ ( Shih and Rothfield, 2006 ).